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Farmers Almanac
The 2014 Farmers Almanac
Farmers' Almanac

It’s Not The Heat. It’s The Humidity and The Dew Point!

It’s Not The Heat. It’s The Humidity and The Dew Point!

Summer is in full swing with an abundance of weather events all across the country. Thunderstorms, rising heat indices, and soaring temperatures are  things you hear a lot during the summer season. We also hear the phrase, “It’s not the heat, it’s the humidity” quite often. This is because while heat certainly takes center stage during the summer months, humidity and dew point readings are really what play a key role in your comfort during the warm weather.

But what are they exactly?

Humidity and dew point are two terms used regularly by your meteorologist during your evening forecast, but they are not interchangeable. They each refer to different things when it comes to measuring moisture in the air. Humidity, or “relative humidity,” measures how close to saturation the air is with water vapor. So if it’s 97% relative humidity outside, that means the air only needs 3% more water vapor to achieve complete saturation of 100% (total air saturation = fog. Many people believe 100% saturation = rain, but fog is what forms in 100% relative humidity).

However, knowing the humidity percentage doesn’t tell the whole story on these summer days. The dew point is a better indicator of how sticky it feels outside. While we may say, “boy, it’s humid out!” chances are, what we’re really feeling is a high dew point. Even if you enjoy the warm temperatures of summer weather, a high dew point temperature –usually anything over 60º Fahrenheit — feels downright uncomfortable for just about everyone.

Meteorologists like to use dew point because it shows a better picture of the relationship of relative humidity to the air temperature. Dew point’s definition is, literally, the temperature at which dew forms, or the temperature the air has to cool at constant pressure to reach total saturation (“fog”).

So that glass of ice cold lemonade that’s leaving a messy ring of moisture on your patio table tells us a lot. It means the ice has actually cooled the glass and the liquid inside down to or below the dew point, so any moisture that’s in the air condenses on the glass. When dew points are low, that glass of lemonade isn’t as sweaty and you’ll notice your towels drying much faster on the clothesline.

While it’s true that the dew point is not a directly measurable parameter, your home weather station can now give dew point readings.

Dew points:

40-50 – Nice and comfortable.
60s – Getting uncomfortable.
70s – Oppressive!

Here’s a temperature, humidity and dew point calculator to play with. Have fun!

Your best defense in the oppressive heat is to ratchet up the fan to high and wait for a thunderstorm to usher in some drier air.

 

9 comments

1 Jan { 07.31.14 at 9:37 pm }

I have rheumatoid arthritis (since my teen years–now 61) and I have found that the dew point is the key to how much pain and inflammation I experience. It is key to how I feel during the summer. Most think that to move to a warm climate is the answer to living with RA but for me, the colder the climate, the better I feel. I belong in Alaska, I think. :o)

2 Deb { 07.31.14 at 8:38 am }

@Darlene,

I use accuweather which provides all you need to know. http://www.accuweather.com/en/browse-locations

just adjust for your location.

3 Calamine L. { 07.31.14 at 12:36 am }

Found this windchill calculator and chart…

http://www.crh.noaa.gov/ddc/?n=windchill

4 darlene macneal { 07.30.14 at 4:52 pm }

HATE, HATE THE HIGH HUMIDITY!!!!!!!!!!!!!!!!!!!!! DEWPOINT—–FOLLOW IT CLOSELY!!! AND THE WEATHER STATIONS RARILY SAY WHAT THE DEWPOINT IS!!!! I HAVE AN OLD SCANNER THAT I LISTEN TO THE WEATHER ETC. ON!!!!!!!!!!!!!

5 Susan Higgins { 07.30.14 at 1:44 pm }

Hi Raymond, I found this formula: Wind Chill = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16)
T is the air temperature in degrees Fahrenheit, and V is the wind speed in miles per hour.

6 Raymond Guinter { 07.30.14 at 11:12 am }

can you explain how they figure the wind chill number? I know it has something to do with wind speed and temp. But how is it calculated?

7 Deb { 07.30.14 at 10:53 am }

The humidity this summer has been miserable to deal with. In the news today parts Newfoundland, Canada are dealing record breaking numbers.

http://www.cbc.ca/news/canada/newfoundland-labrador/temperature-humidex-values-set-to-approach-all-time-highs-1.2722464

“CBC Meteorologist Ryan Snoddon said it’s lining up to be a record-breaking couple of days, with temperatures forecast near 30 C, and a humidex near 40.

“The July 30 record of 27.8 C set back in 1967 is sure to fall today,” he said.

“Tomorrow’s July 31 record of 29.3 C, set back in 2004, will also likely be broken.”

Snoddon said the 30-degree mark at the St. John’s weather station is tough to come by — it has reached that mark just nine times over the past 72 years. “

8 Susan Higgins { 07.29.14 at 4:56 pm }

Actually, John Glenn’s birthday was July 18, 1921, so he just turned 93. But thank you for pointing it out! http://www.biography.com/people/john-glenn-9313269

9 dUe date { 07.29.14 at 3:09 pm }

My Maine newspaper’s almanac failed to mention that today is John Glenn’s 92nd birthday. As a native Ohioan I’m offended by this TYRANNY!

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